STUDIES ON COOLING PERFORMANCE OF ROUND COOLING HOLES WITH VARIOUS CONFIGURATIONS ON A HIGH-PRESSURE TURBINE VANE

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1 A tte Proceedings of ASME Turbo Exo 018 Turbomachinery Technical Conference and Exosition GT018 June 11-15, 018, Oslo, orway ee nd y nl O dea R y o C GT STUDIES O COOLIG PERFORMACE OF ROUD COOLIG HOLES WITH VARIOUS COFIGURATIOS O A HIGH-PRESSURE TURBIE VAE Ken-ichi Funazaki Iwate University Morioka, Iwate, Jaan Fumiya Kikuchi Graduate School of Iwate University Morioka, Iwate, Jaan Takeomi Ideta IHI Co. Yokohama, Kanagawa, Jaan Issei Tashiro Graduate School of Iwate University Morioka, Iwate, Jaan Yuhi Tanaka IHI Co. Yokohama, Kanagawa, Jaan ABSTRACT This study deals with detailed exerimental investigations on cooling erformance of round cooling holes with three hole configurations on the ressure surface of a large-scale test model of a high-ressure turbine vane for gas turbines, where one configuration is a conventional one and the other two are newly designed through a GA (Generic Algorithm) otimization. The urose of this study is to see how those new hole configurations erform under realistic flow conditions. A blow-down tye wind tunnel is used in this study, where IR camera-based transient method is alied to the measurement of film effectiveness and heat transfer distributions on the test model. umerical simulations using a commercial software are also carried out to enhance the understanding of the cooling erformance of each of the hole configurations. cooling holes with comlicated geometry usually costs much. On the other hand, it is quite raid and relatively less exensive to make conventional cooling holes or round cooling holes with devices such as ram (Barigozzi et al. [6]), flow-control devices (Funazaki et al. [7]) or various hole configurations or comound angle by use of advanced laser drilling technology. Comound angle holes have been also widely investigated by many researchers in exerimental and numerical manners (Lee et al. [8], Schmidt et al.,[9], McGovern et al. [10]). Kusterer et al. [11] roosed a combination of two differently angled cooling holes that enabled a drastic increase in film effectiveness. In consideration of those favorable imacts of comound angle of the holes, this study dealt with detailed exerimental investigations on cooling erformance of round cooling holes with three hole configurations on the ressure surface of a largescale test model of a high-ressure turbine vane for gas turbines. One hole configuration was rather a conventional one, consisting of equally-saced identical cooling holes with single comound angle and the other two hole configurations were newly designed ones, being comosed of 6 airs of two differently angled cooling holes in order to achieve better film coverage over the ressure surface. A blow-down tye wind tunnel was used in this study, where IR camera-based transient method was alied to the measurement of film effectiveness and heat transfer distributions on the test model. In order to attain a wide viewable field on the model surface, IR-transarent lastic lates, GAT (Asahi-kasei Engineering Co.) were used along with careful calibration of temerature image catured through GAT. umerical simulations using a commercial software were also carried out to enhance the understanding of the cooling erformance of each of the hole configurations. ITRODUCTIO It is crucial for the imrovement in thermal efficiency of gas turbine to increase turbine inlet temerature of the gas turbine. To achieve this goal, develoing effective cooling techniques is very imortant and therefore a number of relevant studies have been made, esecially on film cooling. Because a conventional round cooling hole tends to exhibit a oor cooling erformance esecially for high blowing ratio due to the effects of counterrotating vortex air (CRVP) (Roshko and Fric [1]), many of them are related to the investigation to control or suress CRVP so as to obtain better cooling air coverage over the target surface. Studies on exit shae of the cooling hole like fan-shaed or diffuser cooling hole have been also executed by many researchers (Goldstein et al., [], Bunker [3], Schroeder and Thole [4], Colban et a., [5]) and fa-shaed cooling holes are nowadays widely alied to gas turbines, while creating such 1 Coyright 018 ASME

2 OMECLATURE A : total area of cooling hole [mm ] total BR : blowing ratio c : secific heat [J/K kg] C : actual chord length [m] C : static ressure coefficient C : axial chord length [m] x dd,, d : hole diameters [mm] 1 h : heat transfer coefficient [W/m K] m : mass flow rate of cooling air [kg/s] c n, n : hole count 1 P : hole itch [mm] S : non-dimensional surface length [m] t : time [s] T : temerature [K] u : average secondary air velocity [m/s] s U : cascade exit velocity [m/s] XY,, Z : axial, itchwise and sanwise directions [m], : static ressure, stagnation ressure [Pa] 0, : angle between the hole axis and the surface [deg] 1, : angle between the hole axis and the main flow 1 direction [deg] : film effectiveness : thermal conductivity [W/K m] : density [kg/m 3 ] Subscrition 1, : (inlet and outlet) or (hole 1 and hole ) f : film temerature j : index of time ste m : main flow s : secondary flow Figure Test section Test Section The test section was laced at the exit of the extended contraction nozzle with the wooden connection duct. The test cascade consisted of two large-scale turbine vanes; one was for the suction side measurement and the other was for the ressure side measurement. Each of the test vanes contained a lenum and cooling holes were machined on the ressure side as shown in Figure 3. Details of the hole geometries are exlained later. TEST FACILITY AD THE MEASUREMET SETUP Figure 1 shows the test facility used in this study, along with a schematic of the test section as shown in Figure. Figure 3 Test models Figure 1 Test facility Since the cascade was comosed of only two vanes, the osition of the vanes relative to the ustream connecting duct was selected with great care in addition to the adjustment of the angles of outlet guide lates so that the measured ressure distribution around the vane matched the rescribed one. To Coyright 018 ASME

3 achieve this goal, the test vanes for the measurement of the static ressure on the suction and ressure sides were manufactured from ABSPlus resin by use of 3D rinting technology(stratasys Dimension Elite) and installed instead of the vanes with cooling holes. Figure 4 illustrates the measured and rescribed static ressure coefficients, which was defined as follows, 0 C, (1) 1 U where and U were a stagnation ressure and cascade exit 0 velocity, measured by a Pitot tube at the cascade inlet and outlet, resectively. It is evident that the measured static ressure distributions were in a good agreement with the calculation obtained by using a commercial software, which will be described in the following. ote that disagreements are also seen in Figure 4, which was robably due to some troubles in manufacturing ressure holes by the 3D rinter. In conclusion, the setting of the cascade was found to be satisfactory for the measurements aimed in this study. The secondary air was sulied by the secondary blower. After assing through the ustream laminar flowmeter, the secondary air temerature was increased with an electric heater box. The heated air then entered the lenum inside the vane and was ejected from the cooling holes. The flow rate of ejected secondary air was determined from the difference of flow rates measured by the ustream and downstream laminar flowmeters. The reason for using two flowmeters was to ensure the sanwise eriodicity of flow attern of the ejected secondary air downstream of the cooling holes. The flow rate of ejected air was adjusted by use of inlet and outlet valves. Hole Configuration Figure 5 shows three tyes of hole configurations examined in this study, which are called Model 1, Model and Model 3. Information on the hole geometries such as hole diameter and hole axis angles are listed in Table 1. ote that the hole counts ( n 1, n ) were (1, 0), (6,6) and (6,6) for Model 1, Model and Model 3, resectively. Model 1, which can be regarded as a reference model, consisted of round holes with the axis laterally inclined by 1, so-called comound angle holes. Model is comosed of two tyes of comound angle holes (, ) with 1 different hole diameters ( d 1, d ). Model 3 is also comosed of two tyes of cooling holes whose axes are directed in the oosite sides with each other. Model and Model 3 were designed through numerical otimizations based on GA (Generic Algorithm). Two hole diameters, hole comound angles, hole itch and hole streamwise distance were selected to be design arameters and a resonse curve was generated on the basis of the numerical results for 50 hole configurations on a flat-late as DOE (Design of Exeriment). The design constraint was a constant total amount of cooling air sulied to one vane, which means the averaged hole area for one hole itch was almost constant. Indeed, the averaged hole area S, defined as av S ( nd nd )/4 P A P, () av 1 1 total was 7.55 for Model and 7.64 for Model 3, resectively, while S was.7 for Model 1 (see Table 1). av Figure 4 Measured static ressure distribution around the test vane in comarison with the calculation Figure 5 Hole configurations Table 1 Detail of hole geometries Hole 1 Hole PP αα 1 ββ 1 dd 1 αα ββ dd Model Model Model nn 1 nn AA tttttttttt SS aaaa Model Model Model Exerimental Aroaches This study emloyed a transient method to determine film effectiveness and heat transfer coefficient. Infrared (IR) cameras (EC Avio H640) and infrared-transarent lates, GAT (Asahi Kasei Engineering), was used in the test section as shown in Figure to cature time-deendent temerature distributions on the test vane. While GAT can rovide wide view for IR camera with low cost, caution is needed about its low infrared transmittance. Therefore, careful investigations were carried out to examine the characteristics of GAT using a custom-made 3 Coyright 018 ASME

4 target object whose surface temerature was electrically controlled. It was found from this investigation that the thermal data from IR camera through GAT was almost indeendent of the angle of GAT while the angle was larger than 45 deg. The transient method adoted in this study is briefly described as follows, while the readers may refer to Kawabata et al. [1] for more detail, including the information on the uncertainty of the measurements. When the test model that can be regarded as semi-infinite body is exosed to time-varying film temerature T, the f temerature rise of the model surface follows Eqs. (3)-(5), T ( t) T H( t )( T ( t ) T ( t )), (3) w i j f j f j 1 j 1 Ht, (4) ( ) 1 ex( )erfc( ) j h t / c, (5) j where T () t is the surface temerature, T is the model initial w i temerature, T T is a small temerature increment of, j, j 1 the main flow, is time lag of the temerature increment from j the starting oint, is the number of small increments. Defining film effectiveness by T T ( t ) m f j, (6) T T( t ) m s j Equation (3) can be re-written using Eq. (6) T ( t) T H( t )( T( t ) T( t )). (7) w i j s j s j 1 j 1 Substituting two different elased instants, t and t a b for t in Eq. (7), the following non-linear equation to determine heat transfer coefficient h is obtained T ( t ) T w a i T ( t ) T w b i j 1 k 1 H( t )( T( t ) T( t )) a j s j s j 1, (8) H( t )( T( t ) T( t )) b k s k s k 1 Finally, film effectiveness is obtained from T ( t ) T w a i. (9) Ht ( )( T T ) j 1 a j sj, sj, 1 Figure 6 is a icture of data maing grid consisting of coer stris. This grid was for transforming the data on the curved vane surface measured by IR camera onto data over a flat lane. The transform function was determined by caturing a thermal image of the grid whose coordinates on the flat lane were known. Besides, the calibration of temerature from the thermal image through GAT was made against the data of thermocoule on the ressure surface of the vane. For this urose, several thermocoules were buried just beneath the model surface, where the surface was coated with black aint. As for the uncertainly analysis, it was found from the revious study by Kawabata et al. [1] that the uncertainly of heat transfer coefficient was 6.8% and film effectiveness was 9.5%. Test Conditions Reynolds number based on actual chord length and inlet velocity was 700,000. Since the secondary air was heated in this study, the ratio between secondary and main air densities was The blowing ratio, which was defined by u m A s s c total BR U U, (10) m m m m was 0.5, 1.0 and 1.5. For each blowing ratio, the measurement was reeated three times. Since the total hole area A differs total for each hole configuration, the blowing ratio eventually differs even when the same amount of cooling air is sulied to the test model. This means that if the amount of cooling air condition were constant, which was the design constraint for Model and Model3, the blowing ratio for Model 3 would be 16.1% lower than that for Model. Figure 6 Grid for data maing UMERICAL SIMULATIO Flow Solver and Domains The flow field around the turbine vane was reroduced by using ASYS CFX Ver as incomressible flow solver, along with the comutational unstructured grid generated by ASYS ICEM Ver in the comutational domain shown in Figure 7. This domain was comosed of three sub-domains for the flows in the main flow field, cooling holes and lenum. In each sub-domain, rism meshes with 0 layers were attached to the wall boundaries, where the nearest node was located so as to make its wall unit y less than unity. Those sub-domains were connected each other by taking full advantages of CFX connection tools. The total number of nodes used in the calculation amounted to about 6 millions. RAS (Reynolds-Averaged avier-stokes) simulation was made using SST (Shear Stress Transort) turbulence model. Besides, according to the manual of CFX, the solver uses secondorder accurate aroximations as much as ossible. 4 Coyright 018 ASME

5 Boundary Conditions All boundary conditions were based on the relevant exerimental data. Inlet velocity and the stagnation temerature were secified on the cascade inlet boundary, while the static ressure was imosed on the cascade outlet boundary. The eriodic boundary condition was alied to the itchwise boundaries. The non-sli and adiabatic boundary condition were used on every solid surface as well as to and bottom boundaries. Besides, the inlet flow rate and the stagnation temerature were secified on the inlet of lenum and outlet flow rate was given on the exit of lenum. reattached the surface so as to increase the film effectiveness thereafter. Figure 7 Comutational domain RESULTS Measured Film Effectiveness Figure 8 shows film effectiveness contours measured on the ressure surface of Model 1 for three blowing ratios, along with the schematic showing the measurement area in this study. In this figure, S reresents non-dimensional distance, which is defined as S S SL, (1) where S is a surface length measured from the most ustream of the cooling holes for each hole configuration and SL is the surface length of the ressure side of the test vane. Those contours were sanwisely averaged, resulting in the averaged film effectiveness distributions along the surface dislayed in Figure 9. ote that the secondary air was sulied from the left-hand side. Because of relatively narrow itch of the cooling holes in this case, the film effectiveness near the cooling holes became almost uniform for BR = 0.5. As BR increased, the film effectiveness just downstream of the cooling holes tended to decrease first, followed by the exansion of film-coverage area in the streamwise and itchwise directions. This trend can be clearly confirmed from Figure 9, showing the shar decrease in the film effectiveness near the holes for BR = 1.0 and 1.5, accomanied by the recovery of film effectiveness. The film effectiveness decrease could be attributed to lift-off, which then Figure 8 Film effectiveness contours on the ressure surface of Model 1 for three blowing ratios 5 Coyright 018 ASME

6 Figure 9 Sanwisely-averaged film effectiveness on the ressure surface of Model 1 for three blowing ratios Figure 11 Sanwisely-averaged film effectiveness on the ressure surface of Model for three blowing ratios Figure 10 Film effectiveness contours on the ressure surface of Model for three blowing ratios Figure 1 Film effectiveness contours on the ressure surface of Model 3 for three blowing ratios 6 Coyright 018 ASME

7 examle, two searated film-covered zones are clearly identified in the simulation of Model for BR =0.5, which tended to merge with each other as the blowing ratio increased, shifting in the sanwise direction. As for Model 3, the lateral sread of filmcovered zone was the narrowest for BR =0.5, quite similar to the exerimental observation. Figure 13 Sanwisely-averaged film effectiveness on the ressure surface of Model 3 for three blowing ratios Figures 10 and 11 are film effectiveness contours and averaged film effectiveness distributions of Model. For BR = 0.5, where the sanwise momentum of ejected air was rather small, two searated film-covered zones therefore aeared downstream of Hole 1 and Hole. Exhibiting lateral shift, these two zones tended to merge each other with the blowing ratio. The averaged film effectiveness, esecially just downstream of the cooling holes, was augmented as the blowing ratio increased, however, the augmentation became rather saturated for larger blowing ratio, robably due to the merger of secondary air jets from the two cooling holes. Figures 1 and 13 demonstrate film effectiveness contours and averaged film effectiveness distributions of Model 3. The sanwise extension of each of the film-covered zones in this case was limited to the area where the two holes were located, resulting in the occurrence of no film-covered zones around the cooling holes, although slight lateral sread of the film-covered zones was observed with the blowing ratio. Because of these characteristics of the cooling hole configuration, the averaged film effectiveness from S =0.0 to S =0.1 was almost indeendent of the blowing ratio. After the above-mentioned area, the averaged value tended to increase with the blowing ratio, as shown in Figure 13. Comarison with the Calculation The authors exerience in RAS simulation and exeriments using IR camera (Funazaki [13]) tells that RAS tends to overestimate the film effectiveness in comarison with the exerimental data, robably due to oor caability in redicting the mixing rocess, in addition to some effects of heat conduction. This is also the case in the resent study, as shown in Figure 14 for Model and Figure 15 for Model 3. In other words, RAS simulation adoted in this study failed to qualitatively reroduce the exerimental film effectiveness, however it aears that RAS simulation was successful in redicting the flow atterns of cooling air to some extent. For COCLUSIOS In ursuing further enhancement of film effectiveness by use of conventional round cooling holes for turbine alications, this study aimed at the exerimental and numerical clarifications of imacts of comound angle methods emloying one conventional and two newly roosed hole configurations, while the latter configurations were develoed by use of RAS-based GA. The cascade tests were executed by using large-scale turbine vanes with different hole configurations to measure their film effectiveness contours. RAS-based numerical simulation was also conducted. The findings in this study can be listed as follows, 1. Model 1, consisting of simle comound angle holes with narrow itch, exhibited a better cooling erformance on the ressure surface of the turbine vane than the other newly roosed models. This was mainly due to the closeness of the cooling holes in Model 1, each of which was searated by less than.5 d. Desite of the favorable film coverage attained by 1 Model 1, its alication to actual turbine vanes requires great caution in terms of the manufacturing cost as well as integrity roblems.. In Model, which was comosed of two different comound angle holes directed towards the same side, the secondary air jets from the holes tended to merge with the blowing ratio, resulting in enhancement of film-covered zone. However, the effects of higher blowing ratio were limited to the regions near the cooling holes and the film coverage far downstream of the cooling holes was not imroved even for the highest blowing ratio. 3. In Model 3, with two comound angle holes directed in the oosite sides, the film coverage downstream of the combined cooling holes increased with the blowing ratio. Desite the fact that the total hole area of Model 3 was about 0% larger than that of Model and BR = 1.5 for Model corresonded to BR = 1.6 for Model 3, it can be stated that Model 3 yielded better laterally-averaged cooling erformance in comarison with Model over S 0.07 region, at least, for higher blowing ratio. However, there remained uncooled regions in Model RAS simulation was successful in redicting the extent of film coverage downstream of the cooling holes for the newly roosed configurations, although there aeared quantitative discreancies between the measurement and the calculation. 7 Coyright 018 ASME

8 Figure 14 Comarison between the exerimental and calculated film effectiveness of Model for three blowing ratios (left: Ex. / right: CFD) Figure 15 Comarison between the exerimental and calculated film effectiveness of Model 3 for three blowing ratios (left: Ex. / right: CFD) ACKOWLEDGMETS The authors are greatly indebted to Mr. Shun Fujita for his contribution to this study. film cooling, Int. J. Heat and Mass Transfer, Vol. 17, o. 5, [3] Bunker, R.S., 009, Film Cooling: Breaking the Limits of Diffusion Shaed Holes, Proceedings of International Symosium on Heat Transfer in Gas Turbine Systems (Turkey) [4] Schroeder, R.P. and Thole, K.A., 014, Adiabatic Effectiveness Measurements for a Baseline Shaed Film Cooling Holes, ASME TURBO EXPO 014, GT [5] Colban, W.F., Thole, K.A. and Bogard, D.A., 011, A filmcooling correlation for shaed holes on a flat-late surface, ASME J. Turbomachinery, Vol. 133, o. 1, [6] Barigozzi, G., Franchini, G., and Perdichizzi, A., 007, The effect of an ustream ram on cylindrical and fan-shaed hole film cooling art1: Aerodynamic results, ASME Turbo Exo 007, GT [7] Funazaki, K., akata, R., Kawabata, H., Tagawa, H. and Horiuchi, Y., 014, Imrovement Of Flat-Plate Film Cooling Performance By Double Flow Control Devices: Part I Investigations On Caability Of A Base-Tye Device, ASME Turbo Exo 014, GT [8] Lee, S. W., Kim, Y. B. and Lee, J. S., 1995, Flow Characteristics and Aerodynamic Losses of Film cooling Jets with Comound Angle Orientations, ASME-aer 95- GT-38 [9] Schmidt, D.L, Sen, B., Bogard, D.G., 1996, Film Cooling with Comound Angle Holes: Adiabatic Effectiveness. ASME J. Turbomachinery, Vol. 118, [10] McGovern, K.T., Leylek, J.H., 000, A Detailed Analysis of Film-Cooling Physics: Part II, Comound Angle Injection with Cylindrical Holes, ASME J. Turbomachinery, Vo. 1, [11] Kusterer, K., Elyas, A., Bohn, D., Sugimoto, T. and Tanaka, R., 008, Double-jet Film Cooling for Highly Efficient Film-Cooling with Low Blowing Ratios, ASME Turbo EXPO 008, GT [1] Kawabata, H., Funazaki, K., akata, R. and Takahashi, D., 014, Exerimental and umerical Investigations of Effects of Flow Control Devices uon Flat-Plate Film Cooling Performance, J. Turbomachinery, Vol. 136 [13] Funazaki, K., 017, Enhancement of Film Effectiveness of Cooling Holes with Fan-Shaed Exit Geometry by the Alication Double Flow-Control Devices: Otimization in Consideration of Device Offset, ASME Turbo Exo 017, GT REFERECES [1] Roshko A., Fric T.F., 1994, Vortical Structure in the Wake of a Transverse Jet, J. Fluid Mechanics, Vol. 79, [] Goldstein, R.J., Eckert, E.R.G. and Borggraf, F., 1974, Effects of hole geometry and density on three-dimensional 8 Coyright 018 ASME